For many years activated slutch has been thickened by passage through a settling tank such that the residence time in the tank is of the order of 24 hours. This technique, besides requiring much time and large equipment, has the disadvantage that the thickened sludge will have a solids concentration of only 1.5 to 2.0% by weight.
With a view to obtaining higher levels of solids in the concentrated sludge, various techniques have been developed utilizing sophisticated equipment such as floatation apparatus where suspended solids contained within the sludge are carried to the surface by small bubbles of gas produced by electrolytic action or out of solution.
Such floatation apparatus is effective in removing water from the sludge so that the concentrated sludge will have 5% solids by weight or thereabouts. This technique has a number of disadvantages in that the equipment used is costly to purchase and maintain and that the processing costs are high.
The applicant has conducted an extensive research program with a view to finding an apparatus of thickening sludge to have a concentration of the order of 5% by weight of solids with simple equipment and in an economical manner.
The present invention is based upon the discovery that if the sludge is dosed with a cationic polyelectrolyte and then handled very gently, it will become relatively free draining.
According to the present invention, the apparatus for thickening a sludge comprises a structure for dosing the sludge with a cationic polyelectrolyte, allowing time for the development of a floc structure, placing the sludge onto a surface through which liquid may drain freely in such a manner that the floc structure is retained and removing the thickened sludge from said surface.
The invention will be further apparent from the following description with reference to the figures of the accompanying drawing which show, by way of example, only one form of apparatus embodying the invention.